Author
Topic: Rigol DS1052E nasty surprise! (Read 96645 times)

I was impressed by the DS1052E I had recently purchased and I got myself a brand new one. But, alas, it seems that the new device has some serious issues my previous one did not have; neither I have the older DS1052E in my possession anymore...

Explaining myself: The new DS1052E seems to have a higher noise floor than the older one and whenever any probe tip touches the ground of the device, the display is instantly filled with some kind of very high frequency sinusoidal noise in the range of 95MHz, making practically unable any measurements to be done; especially in the lower voltage ranges.

For example, the older device could easily read the noise of its own PSU, by simply touching the probe tip to the front USB +5V line, and it showed clearly the PSU oscillator pattern without the need of any kind of signal filtering; even with the probe's ground lead floating. The new one is totally unable to take the same reading because every time the probe tip touches the ground (or the +5V line) the aforementioned 95MHz noise covers any possible input signal; input filtering does not really help. I cannot even read the 1.0KHz probe calibration output signal without some serious filtering. Even with the use of the probe ground spring, the device does not return the clear-cut waveform my previous instrument did. I do not know what is wrong but I suspect that there is some kind of a hardware issue.

I will let the snapshots speak for themselves:

Noise 1: Probes disconnected

Noise 2: Ch1 probe connected. Notice the frequency counter.

Noise 3: Ch1 probe-tip grounded through probe's GND lead. 95.8MHz?

Noise 4: Ch1 probe-tip grounded through aluminium foil. 12.47MHz?

Noise 5: Ch1 probe on the probe calibration output: Unacceptable!

Noise 6: Ch1 probe on the probe calibration output, filtered. Still fuzzy for such a clear signal...

After a brief search, I found out that this is not an isolated case, since another brand new DS1052E had the same exactly symptoms a year ago; so the issue is independent of the new ADC markings (see below). Since I have not purchased the instrument from a Rigol authorised dealer, I do not expect to have it replaced; so, I opened it in hope of finding something that can be fixed.

The mainboard (ver. 0.2 2008/05, build 11/03) is a multi-layer PCB (the bottom layer is marked as sixth and the fifth layer mark is visible beneath it, so it is a three --at least-- layer PCB); and this fact alone rules out any serious attempt to check the PCB for any possible routing faults. So, I begun searching for inconsistencies.

It is interesting that, though the AD8370 and the LMH6552 within the analog front-end shielding retain their diacritics grinding-out, this is not the case for the ADC chips anymore! The five ADCs proudly present their identity now, being devices manufactured from AD and waving their obscure identity in three lines:

RAD0182A #10321924219.1I suppose that the second figure is the mfg. time-code (32nd week of 2010).

DS1052E new ADC markings!

Of course, I guess that if I purchased those quantities of the AD9288 ADC chip Rigol does from AD, the latter ones would be happy to put a picture of my face onto the chip instead of the traditional boring identification marks!

With the help of a bit of *ahem* reverse engineering I drew the schematics of the PSU (sorry I got lazy and did not fire up the CAD-CAM), which seems to be functioning as expected (according to the biasing component values):

DS1052E PSU schematic. (Go to the last picture at the end of the message for a high resolution image of the schematics)

Taking a few readings, the PSU generates the following:1. The main supply, which is 6.40V and powers almost everything.2. A 3.375V supply for the logic section, which is generated from the 6.4V by an LM317T.3. A 15.0V supply for the analog section.4. A negative supply of -12V for the analog section and the cooling fan.

Now, why the main supply is 6.4V? That is simply because the 5.0V partial supplies can be accurately generated locally on board, independently of the long wiring/routing losses.But, why this peculiar voltage of 6.40V exactly? Well, because this specific voltage is what exactly required by the LCD module backlight!

Anyway...Any thoughts or suggestions on the unacceptable noise issue the brand new device has are welcome.

-George

P.S. By the way, it is fun to be right! I have previously speculated that the actual power consumption of the instrument would probably lie between 10..15W. Well, these are the actual readings I took from the PSU of my unit:A: 6.40V x 625mA = 4000mWB: 3.40V x 1195mA = 4063mWC: -12V x -335mA = 4020mWD: 15V x 21mA = 315mWE: LCD backlight: 5.98V x (447mV/5.6Ω) = 477mWTotal power consumption (by the instrument ONLY, without the PSU losses): A+B+..+E = 12.87W!Well, yes, it is fun to be right, indeed!

alm

I would ignore results with a probe connected for now since they might just as well be caused by RFI from some other piece of equipment and/or poor shielding of the probes. For example, clipping the probes ground lead to the probe tip can actually be a fairly good ESD detector. This appears to be what's happening in your noise 3 snapshot (not necessarily ESD, but some sort of inductive pickup). You'll likely see the signal change as you reduce the loop area (eg. compress the loop). I would also ignore counter values for signals like noise 1 and 4, which lack any well-defined trigger point. Noise 1 doesn't look that bad to me (but I don't have an DS1052E to compare). Is it outside of the specs given in the manual? Does shorting the input (eg. with a 50 ohm terminator, not a piece of wire) help? If the signal is OK without probe connected at all vertical attenuator settings, I think you can exclude the signal acquisition circuit.

Noise 5 looks bad to me. Can you verify whether the signal is actually clean with another scope? Can you try different probes? Try a simple coax cable to a (fairly clean) signal generator? Try the same tests in a different environment with less electromagnetic fields present? Something like an electronic ballast for a fluorescent light can generate a lot of RFI.

I was impressed by the DS1052E I had recently purchased and I got myself a brand new one. But, alas, it seems that the new device has some serious issues my previous one did not have; neither I have the older DS1052E in my possession anymore.....Total power consumption (by the instrument ONLY, without the PSU losses): A+B+..+E = 12.87W!Well, yes, it is fun to be right, indeed!

1. The main supply, which is 6.40V and powers almost everything.2. A 3.375V supply for the logic section, which is generated from the 6.4V by an LM317T.3. A 15.0V supply for the analog section.4. A negative supply of -12V for the analog section and the cooling fan.

Now, why the main supply is 6.4V?

I would've thought the LCD backlight would require a constant current supply? If this is true disconnecting it would be a bad idea, as the voltages around the circuit would rise but this does seem like an odd way of doing it.

I am very aware of the fact that the probe ground lead connected to the probe tip forms an inductive pickup head. But the new device is standing at the same exactly location of my desk/bench the older one was. I even powered off the PC monitor (a DELL U2410 --not some el-cheapo EMI generator that produces images!) the DSL modem and the DECT phone that are located a few meters away: Nothing changed! Additionally, there are no CFL lightning devices in my place (I really hate them!) but only halogen ones --even in the bathroom! The only source of EMI is my own body; but wearing an ESD wrist strap the interference is minimised, as it can be seen on the "Noise 2" figure, above, where a few millivolts p-p of the 50Hz hum my own body was picking up were added to the VHF internal noise of the device.

I do not have a proper 50Ω terminator in hand, neither a male BNC to construct one. But I terminated the scope's BNC input using a 51Ω resistor with its leads trimmed as short as possible and, as a result, the residual HF noise of 7.20mVpp (as seen on the "Noise 1" figure, above) was instantly doubled!

The problem is that my previous unit did not have any of those issues. I could just probe any test PCB and read nice, noise-free waveforms. Using a better (lower noise, x100) probe I had even sharper edges and cleaner lines. But, this specific instrument seems to have some internal loop and/or self-oscillation and/or saturation problem that is amplified on the presence of any external test signal.

Anyway, thank you both for your thoughts!_____

scrat,

Yes, proper grounding is one of the first steps I always take._____

sat,

I took those current consumption readings by inserting four 100mΩ shunt resistors directly in series to the four DC outputs of the PSU PCB connector.

Having in hand those output current figures and consulting the PSU schematics, it is very easy to calculate the total thermal losses of the rectifiers and the regulators at the flyback secondary side of the PSU. For example, the power dissipated on the 317T is equal to:(6.40V - 3.40V) * 1195mA = 3585mWAssuming that the Vf of the Schottky rectifier (the Tunnel diode, according to the schematics!) is 0.9V (my DS1052E is unable to take that reading in its present state...), then the power dissipated on the rectifier will be equal to:0.9V*(625mA+1195mA+(447mV/5.1Ω)) = 1722mWIn the same manner, assuming that the Vf of the MUR460 rectifier is 1.1V, it dissipates:1.1V * 335mA = 369mWUp to this point, the major losses of the secondary side of the PSU are:3585mW + 1722mW + 369mW = 5.7WThe partial power consumption level is now at 18.54W, without having included the losses of the primary side of the PSU, which is trickier to calculate since there are inductance losses at the flyback. But we can estimate those losses by subtracting the calculated partial power results of the secondary side (12.87W + 5.7W = 18.6W) from the measured consumption (which in my case was 23VA) to have another four and a half Watts estimated power loss, which is a very reasonable value for the specific stand-by PSU setup that does not have a PFC stage!_____

Hero,

If you open the link of the LCD module datasheet in my first message, you will see that the backlight is consisted of 14 LEDs that are connected in seven parallel arrays of two LEDs in series each (thus, 6.4V=2*3.4V).

In the PSU schematics I drew, the backlight connector is supplied by the 6.40V rail through a high-side P-Ch MOSFET switch in series to a 100μH inductor of 0.3Ω and a 5.1Ω current limiting resistor.

So, the LCD baclight is not actually powered directly by the 6.4V rail but by a proper current source.

-George

« Last Edit: June 13, 2011, 08:22:22 pm by A Hellene »

Logged

Hi! This is George; and I am three and a half years old!(This was one of my latest realisations, now in my early fifties!...)

In lack of a precision waveform generator, I took some readings of a mega16 test circuit, which was battery powered in order to exclude any possible ground loop noise, since its ground is floating. I chose to read a simple waveform generated from a mega16 because these Atmel microcontrollers are notorious for their output stage drive capability, since their output stage is a true complementary one with Rds_ON=24Ω ±10% at Vcc=5.0V at room temperature.

Here are some readings from a mega16 running at Vcc=5.02V, programmed to generate a simple 1.038MHz, 50.0% duty cycle square wave, using its hardware PWM and clocked by its internal RC oscillator calibrated for optimum UART baud-rate.

Please, keep in mind that using the same exactly mega16 setup, my previous unit displayed a crisp square wave without the need of using the ground spring or any further filtering.

1: Ch1 x1 waveform using ground lead, unfiltered

2: Ch1 x10 waveform using ground lead, unfiltered

3: Ch1 x10 rising edge using ground lead, unfiltered

4: Ch1 x10 waveform using ground lead, filtered

5: Ch1 x10 rising edge using ground lead, filtered

6: Ch1 x10 waveform using ground spring, unfiltered

7: Ch1 x10 waveform using ground spring, filtered

Here is a couple of readings using a lower noise x100 probe (250MHz, 6.5pF).Please, note that the x100 probe is properly compensated, using the 1.0KHz probe compensator connector.

In lack of a precision waveform generator, I took some readings of a mega16 test circuit, which was battery powered in order to exclude any possible ground loop noise, since its ground is floating. I chose to read a simple waveform generated from a mega16 because these Atmel microcontrollers are notorious for their output stage drive capability, since their output stage is a true complementary one with

I will be glad to repeat the test for the second channel alone, even though I do not expect to see any differences at all... I suspect that the second test outcome will be exactly the same to the first one because I think that the problem must not be located to a single analog front-end channel.Anyway, I have already powered up the device to warm up and I am going to do the test using the same probes, in order to rule out any possible differences due to probe-related deviations.

After figuring out the PSU of the device I also visually inspected the mainboard, expecting to see some sort of a missed soldering or an accidental short or something unusual; but I do not have any documentation related to that instrument so I can only go on through the deductive method.

On the other hand, the "Noise 8" screenshot at the first message indicates some sort of an internal self-oscillation at 100MHz and I am not sure where it comes from. I will try to recapture that, using dots instead of vectors to display the actual ADC samples; in a hopeful manner, this will shed some light to the ADC sampling section.

-George

« Last Edit: June 14, 2011, 10:05:09 pm by A Hellene »

Logged

Hi! This is George; and I am three and a half years old!(This was one of my latest realisations, now in my early fifties!...)

Well, it seems that the Channel 2 test results are a carbon copy of the Channel 1 ones, above!So, the analog front-end alone must not be held responsible for the noise issue.

Check: Two down (PSU, Analog front-end), who-knows-how-many more to go!

Here are the Channel 2 test results:

1. Ch2 x1 waveform using ground lead, unfiltered

2. Ch2 x10 waveform using ground lead, unfiltered

3. Ch2 x10 rising edge using ground lead, unfiltered

4. Ch2 x10 waveform using ground lead, filtered

5. Ch2 x10 rising edge using ground lead, filtered

6. Ch2 x10 waveform using ground spring, unfiltered

7. Ch2 x10 waveform using ground spring, filtered

Changing the probe:

8. Ch2 x100 waveform using ground lead, unfiltered

9. Ch2 x100 waveform using ground lead, filtered

Something different, now:

10. Ch1 probe on the probe calibration ground; 1GSa/s. Similar to "Noise 8" at the first message but in dot mode.

All the individual ADC samples are in the most expected positions.Note that this is a full 1GSa/s because Ch2 is off as well as the long memory option; so, all the ten ADCs are involved and they all seem to be in order.

I've always wondered if some of the scopes sold via 'unauthorized' channels are actually counterfeit, as suggested by this note by Rigol to the public. I haven't heard of any counterfeit warnings from Rigol; the main problem is unauthorized channels but it doesn't explain what this letter means in regards to ...' some products from illegal sales channels are not even originally produced by Rigol Technologies, Inc.' which suggests to me its contract factories are producing excess units for sale outside of Rigol's control and could lead to quality control issues.

No one until now managed to present a counterfeit Rigol oscilloscope, not even Rigol. And I mean not just pointing at one and shouting 'counterfeit!', but showing evidence. Don't you think Rigol would publish information on how to distinguish an original Rigol from a counterfeit product if they have that information?

That letter just deploys scare tactics. Rigol got flack from their authorized resellers regarding the wave of gray imports and was forced to do something. Instead of stopping to supply the "illegal" Chinese exporters with oscilloscopes they wrote that letter, exaggerating things. E.g. already the use of the word "illegal" is an exaggeration.

How much can you trust that letter? Not much. Read the sentence carefully. They claim

"Some products sold ... are not even originally produced by Rigol Technologies, Inc."

They didn't dare to claim

"Some products sold as Rigol products ... are not even originally produced by Rigol Technologies, Inc."

See the difference? The original claim is a trivial statement. A vendor might of course sell other products than Rigol products. And why should these products be produced by Rigol? Second, the claim would also apply if Rigol would rebadge OEM products. These would of course also not be "originally produced by Rigol".

The probe calibration screenshot you posted above (300mVpp @ 1.0ms period --probably due to Ch1 probe setup mistakenly set x1 instead of x10) is very similar to what my previous unit used to display. More on the DAC/PCB comments you mentioned, below.

On the aforementioned Rigol statement, yes, I am already aware of it; but I cannot take it seriously. BoredAtWork explains that in a very rational manner._____

BoredAtWork,

I will fully agree with you on the scaremongering tactics any company loosing revenue will use.I also enjoy being meticulous, meaning being careful and precise about the little details, as well as reading between the lines, in any situation!_____

Anyway, being suspicious that the excessive noise problem of my device is probably a result of a PCB or a voltage regulator issue, I begun mapping out the mainboard, which most probably is a six-layered PCB; four layers are already visible and the bottom layer is marked as the sixth one! First step is power routing.

Please, excuse my ASCII art; but this it the fastest way for me to figure out something new to me, since working in anything new, everything is firstly constructed in a temporary place in my mind.

Anyway, this is the progress I have made so far. Mind you, this is a very early stage phase; this is an incomplete piece of work! Well, here it goes:

Well, I guess that after this has been started, Rigol will either sue my ass (even though I am not subjected to any NDAs) or they will replace my faulty unit in order to stop me from figuring out and making public their trade secrets!

--George

« Last Edit: June 16, 2011, 12:31:02 am by A Hellene »

Logged

Hi! This is George; and I am three and a half years old!(This was one of my latest realisations, now in my early fifties!...)

I am not sure if you have contacted Rigol at all, but I contacted them about finding a local distributor for their scopes and when I replied I mentioned this post and just my general concern - they replied with the following - It may be of some use/info

Quote

Hi James,

Very appreciate your sharing of this article with us, and we already know this case, at the same time our R&D attached highly importance to this issue, will find if it's a quality control problem or something else.

We always try every efforts to improve ourselves as to provide most high quality and performance product and service to customers, and would like to have more feedbacks from your side in the future.

It sounds like many of the answers we got from a Chinese supplier for some equipment (Radio Transmitters) we bought at my last job.After a while,you realise that they are "just being nice" & have no real intentions of doing anything about the problem.

They did try,I suppose, when they first arrived,none of them worked,& after a while,they said "Send 'em back."On their return,they sort of worked!Eventually we fixed most of the problems ourselves, most of which were "Quality Control"(or lack of) problems.

They have been replaced now,& the Boss has scheduled a "sledgehammer party" so everybody can take out theirfrustrations on the @#$%$#^&&&^%$!! things.

I just looked at my unit, bought last January from Dealexcel, so directly from China to Europe I successfully made the hack to 100MHz, more for the fun than for utility And I don't really understand the letter from Rigol staff, for me it's obvious my DSO is a genuine Rigol, even the package was "Rigol printed"... Only the price would have been (very !) different if I had bought here !

What I can say about your screenies, compared to my unit :noise 1 & 2 seem normal to me as probe is not grounded, though I get a <5Hz as frequency and 2.40mV max Vpp, no weird frequency...noise 3, 5, 7, 8 : I can't reproduce, I mean I don't know how to get into these, strange...When I connect a probe to the calibration output I immediately get a nice rectangular signal, same values for rise time and Vpp, but I have a very stable 1.00000KHz as frequency. Same as noise 6, but I think a bit less noisy on horizontal lines...

So it seems something is going wrong with your scope.A stupid question (but I am often stupid with my own machines) : you did the calibration process right ? May be you could recall the factory settings then redo a calibration...But for me everything appears to work as something was poorly grounded somewhere...

Well, yes, I have run the auto-calibration procedure; but it is irrelevant because the problem is clearly in the hardware: Some of the screenshots indicate that there is either a PCB or a component failure. I am just trying to locate the problem and this is not easy without proper documentation (schematics, test points references, etc.).

On the other hand, this is not the first DS1052E I lay my hands on, so I can say with a fair amount of certainty that my unit is a legitimate Rigol one; it came with all the paperwork (user's manual, warranty, calibration certificate, etc.) and drivers in a Rigol packaging, just like the previous one I had that was functioning flawlessly. It has just not been purchased from the official Rigol distribution channels for Europe.

-George

Logged

Hi! This is George; and I am three and a half years old!(This was one of my latest realisations, now in my early fifties!...)